While lack of space forbids an explanation of the causes
which would perfect an ice period in the northern hemisphere, I will
say that it could be mainly brought about through the independent
circulation of the arctic waters, which now largely prevent the
tropical waters of the North Atlantic from entering the arctic seas,
thus causing the accumulation of ice sheets on Greenland. But before a
northern ice period can be perfected, it seems that it will need to
co-operate with a cold period in the southern hemisphere; and in order
to have the ice of a northern frigid period melt away, it would
require the assistance of a mild climate in the high southern
latitudes.--_Science_.
* * * * *
AMMONIA.
In the majority of refrigerating and ice machines ammonia gas is the
substance used for producing the refrigeration, although there are
other machines in which other material is employed, one of these being
anhydrous sulphurous acid, which is also a gas. Ammonia of itself is a
colorless gas, but little more than one half as heavy as air. In its
composition ammonia consists of two gases, nitrogen and hydrogen, in
the proportion by weight of one part nitrogen and three parts
hydrogen.
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